Thursday, July 28, 2011

Earth Science Picture of the Day

I had the Earth Science Picture of the Day for today, Thursday, July 28.  Cool!

I'll get back to blogging soon, have been taking a short hiatus.

Tuesday, July 19, 2011

Moon or Afghanistan

Taking a one day break from my Movements of the Moon series.

On a semi-related note, here's a true and depressing cartoon from The Pain Comics.  Fucking al Quaida.

Monday, July 18, 2011

Movements of the Moon - Part V

Links to Part I, Part IIPart III, and Part IV of this series.

Far, far, far away from our little solar system are the visible stars of our local neighborhood in the Milky Way galaxy.  As you go outside at night, and look up at the starry sky (to be fully human, you should do this periodically), you'll see random scatterings of stars and your mind will automatically want to arrange them in patterns.  Humans have done this since time immemorial and created what we today know as constellations.  Modern astronomy recognizes 88 constellations (some we can't see here in the Northern Hemisphere), most named from ancient Greek mythology.

All of the planets basically orbit the Sun in the same plane which is called the ecliptic.

When we look up at the night sky, any planets, if they're visible, will always lie along the ecliptic.  Here's what you would have seen in Ulster County, for example, in early May, 2002 just after sunset.

The red line is the ecliptic and the five naked-eye planets were visible in the sky that evening - Mercury, Venus, Mars, Jupiter, and Saturn.  You always look for the planets in the region of the sky where the ecliptic runs; you'd never look for a planet in the constellation of Orion, for example, because the ecliptic doesn't run through Orion.

The part of the sky where the ecliptic occurs was long ago (we're talking ancient cultures of Meospotamia thousands of years ago) divided up into roughly equal regions signified by 12 distinct constellations - Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra Scorpio, Sagittarius, Capricorn, Aquarius, and Pisces.  The zodiac (modern astronomers recognize these constellations today even though they totally reject their astrological roots.

We call this the zodiac, derived from the Greek ζῳδιακὸς κύκλος (zōdiakos kuklos) or "circle of animals".  During different parts of the year, we see different zodiacal constellations in the night sky because we're in different parts of the Earth's orbit.

OK, what does this have to do with the Moon?  Well the Moon orbits the Earth a few degrees off the ecliptic (we'll talk more about this in another post) so the Moon also appears each night in one of the zodiacal constellations.

As an example, the following figures (click to enlarge) show the position (and phase) of the Moon on July 10, 12, 14, 16, and 18 of 2011 in the southern sky as seen from Ulster County, NY at midnight.  The red line is the ecliptic, the plane in which all the planets orbit the Sun (almost).  Note that the Moon moves from slightly below the ecliptic to slightly above the ecliptic for reasons we'll talk about another day.


Over the course of about a week the Moon moves through the zodiacal constellations of Libra, Scorpius, Sagittarius, Capricornus, and Aquarius.  Over the course of a sidereal month (one orbit of the Moon around the Earth with respect to the distant stars), the Moon will move through all 12 of the zodiacal constellations.  We can't see it in some of them because when the Moon is out during the day (near its New Moon phase), we can't see the stars.  On August 1, 2011, for example, the Moon will be in Leo but rising and setting with the Sun so completely invisible.

Some ancient cultures kept calendar time by the Moon's 27.32 day sidereal movement through the zodiac instead of the by the 29.52 day synodic cycle of phases.  Ancient Chinese astronomers, for example, divided the zodiac into four regions - azure dragon, black tortoise, white tiger, and vermilion bird.  Each of these was then subdivided into 7 "mansions" giving a total of 28 "mansions".  The Moon basically passes through a lunar mansion each day.

Next time, we'll talk about the rising and setting times of the Moon.

Sunday, July 17, 2011

Movements of the Moon - Part IV

If you haven't already, read Part I, Part II, and Part III of this series first.

To date, we've now discussed phases of the Moon, the synodic month (a lunation or cycle of phases), and the sidereal month (an orbit of the Moon around the Earth).  Today, I'd like to discuss a couple of miscellaneous concepts.

The image at right shows the phases of the Moon from Waxing Crescent, 1st Quarter, Waxing Gibbous, to Full Moon and then back to Waning Gibbous, 3rd or Last Quarter, Waning Crescent, and New Moon (the blank space in the lower-right corner).

Note that the features you see on the face of the Moon never change.  In other words, the same side of the Moon is always facing the Earth no matter what the phase.  This is called synchronous rotation and is due, of course, to the strong gravitational attraction between the Earth and Moon.

The side always facing away from the Earth is called the far side and wasn't even seen by humans until imaged by the Russian Luna 3 probe in 1959.  As I mentioned in a previous post in this series, it's sometimes called the dark side but the word "dark" is used only in the sense of it being unknown (at the time), not an absence of light.

The so-called dark side of the Moon is fully lit by the Sun for two weeks each month.  A New Moon phase, when the Moon is between the Earth and the Sun, has the far side of the Moon in full sunlight while the near side that faces the Earth is in darkness (as illustrated by our old friend, the diagram below).

The interesting thing about the far side of the Moon, is that it's very different in appearance from the near side.  The near side has more mare (pronounced "mar-ey" - Latin for "seas") - large volcanic lava plains - and the far side is heavily cratered.  I'll discuss why another time.

Another interesting fact about this tidal locking of the Earth and the Moon is that the Moon "rocks" back and forth a little bit in its orbit allowing us to actually see a bit more than half (59%) of its surface from the Earth.  This is called lunar libration.  The image below illustrates this, both show photos of the waxing Moon at two different times.  The red dot denotes Mare Crisium ("Sea of Crisis") near the Moon's eastern edge.  See the slight difference in position from lunar libration?

Speaking of the size of the Moon, perhaps I should say a few words about the Moon illusion.  When a Full Moon is rising on the eastern horizon it often looks huge - much larger than it appears when higher in the sky.

It's an optical illusion.  You can photograph the rising Moon and establish that it's size really doesn't change - it just appears large near the horizon (the color change is due to blue wavelengths being scattered out near the horizon - leaving red wavelengths to get through - since you're looking though a thicker layer of the atmosphere).

The Moon is actually surprisingly small in the sky.  It covers 1/2° - for reference, the distance from the horizon to the zenith (the point directly over your head) is 90°.  That's means you hold your arm outstretched, extend your pinkie finger, and the fingernail on your pinkie will completely cover the Moon (even when it's on the horizon).  Don't believe me?  Go out at night and try it.

Why do we perceive the Moon larger on the horizon?  No one's quite sure (or, maybe I should say, lots of people think they're sure but they offer competing hypotheses).  Here's a summary of ideas from Dr. Donald Simanek, a retired physics professor.

Enough for today.  The topic for tomorrow, which I briefly mentioned yesterday, goes back to the sidereal month and the Moon moving through the constellations.

Saturday, July 16, 2011

Movements of the Moon - Part III

If you haven't already, read Part I and Part II of this series first.

Today's post will explain the difference between the Moon's synodic month and sidereal month.  The synodic ("with the Sun") month, as already discussed, is the cycle of lunar phases (a lunation) which lasts approximately 29.53 days.

Just for kicks, I decided to look at all the New Moons from January 1, 2000 to January 1, 2025 - a 25 year span of time.  I can do this easily because I have a program called Mica (Multiyear Interactive Computer Almanac) from the U.S. Naval Observatory.  In that span of time, there are 310 New Moons.  That gives, on average, 12.4 lunations each year (which is why traditional lunar calendars, like those used by almost all ancient cultures, always had to be fudged - there aren't an even number of lunar phases in a year).

If I look at the length in time between each adjacent New Moon, I get an average interval of 29.531314 days with a maximum lunation of 29.8242 days from December 18, 2017 to January 17, 2018 and a minimum lunation of 29.2822 days from May 25 to June 24, 2017.  That's a difference of 0.542 days (around 13 hours).

It's interesting that 2017 comes up for both the maximum and minimum lunations (it's also the year of a solar eclipse visible in the U.S.).  Maybe we'll see why when looking at the elliptical orbit of the Moon in a future post (I don't really know why as I write this right now).

Given that a cycle of phases lasts 29.53 days, you might think that's how long it takes the Moon to orbit the Earth, especially given the figure below we've discussed in previous posts.

Turns out that when you look up the period of the Moon's orbit around the Earth, you get a completely different number - 27.321661 days (27 days, 7 hours, 43 minutes, and 11.5 seconds).  A two day difference!  What's up with that?

Keep in mind that the synodic month (one lunation) is the time it takes from New Moon to New Moon - the time when the Moon is exactly lined up between the Earth and the Sun as seen above.  But the image above is static.  Both the Earth and the Moon are orbiting the Sun and that's where the difference comes in.  Let's look at the image below:

Here we'll look at the length of time from Full Moon to Full Moon (same as New Moon to New Moon).  We need some way to mark the Moon's revolution around the Earth.  The easiest way to do this is to mark the Moon's position in the sky with respect to some distant star (they're all distant!).  The image above uses Regulus in the constellation of Leo as an example.  Why a distant star?  Because they're so far away from the Earth that they don't shift visibly in the sky no matter where the Earth and Moon are in their orbits.

Let's suppose, from the perspective of us on Earth, that the perfectly full Moon lines up with Regulus.  What we'll see is that in 27.32 days, the Moon will once again line up with Regulus so we know it's made one complete revolution around the Earth.  This is called a sidereal month, from the Latin word sidus which means "star".

But, after this one revolution with respect to the distant stars, it's not a Full Moon yet!  Why?  Because the Earth-Moon system has also been moving almost 1/12 of the way around the Sun and the geometry has changed.  The Moon has to travel just a little bit more in its orbit (almost 2 days) to reach the position where it's directly opposite the Sun.

Here's a nice animation.

That's why the synodic ("with the Sun") month is 29.53 days long and the sidereal ("with the stars") month is 27.32 days long.  Think that's bad, there are other types of "months" we'll talk about shortly as well.

Some ancient cultures actually marked time with sidereal months instead of synodic months.  Look at the image below.  It shows the Full Moon after midnight on July 15 from Ulster County, NY.  The Moon is in the constellation of Sagittarius.

Twenty seven days later, about one sidereal month, on August 10, the Moon will also be in Sagitarrius.  But the Moon's not full yet.

The Full Moon occurs one synodic month after July 15 on August 13.  But now the Moon is in the constellation of Capricornus.

What some cultures did was watch the Moon move through the zodiacal constellations and then use this to keep track of calendar time.  More on this tomorrow...

Friday, July 15, 2011

Movements of the Moon - Part II

So, in yesterday's post, we discussed how the Moon goes through a cycle of phases - New, Waxing Crescent, 1st Quarter, Waxing Gibbous, Full, Waning Gibbous, 3rd Quarter, Waning Crescent, and New again - in 29.53 days. Today we're going to see why it does this each synodic month.

Let's look at the diagram below - the phases of the Moon are entirely due to the geometrical relationship between the Earth, Moon, and Sun. By the way, contrary to popular misconception, the phases of the Moon ARE NOT due to any shadow from the Earth on the Moon (lunar eclipses are, but that's a different story).

In this diagram, the Sun is off to the right.  Let's imagine we're looking down from above the North Pole of the Earth (the Earth really isn't pictured that way in the image, but ignore that!).  Also ignore the fact that the Earth and Moon sizes are a bit off (the Moon's diameter is about 1/4 the diameter of the Earth) and that their relative distances here are way off (if the Moon and Earth were scaled to the size of a tennis ball and a basketball respectively, they'd be 24 feet away from each other!  Read this for more information).  In other words, the diagram is NOT to scale.

Also in the diagram above, note that the side of the Earth facing the Sun is lit and the side facing away from the Sun is not lit.  Daytime and nighttime on planet Earth.  Now imagine the Earth rotating counterclockwise (from above the North Pole) on its axis once every 24 hours so someone standing on the Earth will travel from night past the terminator (the line between dark and light) into daytime (they'll see the Sun rise in the eastern sky).  The Sun will then rise higher in the sky until it's over their head and then sink into the west and the person travels on the rotating Earth back toward the opposite terminator.  The Sun will set on the western horizon as the person rotates back into the dark half of the Earth and will be back where they started after the 24-hour day. 

For now, we'll ignore the fact that the Earth's axis is tilted and that's why there are more than 12 hours of daylight for us here in the mid-latitudes during the summer and less than 12 hours of daylight in the winter.  We will come back to this later because the Moon's orbit around the Earth is a bit tilted too!

Back to the diagram of the Moon orbiting the Earth.

Similar to the Earth, the side of the Moon facing the Sun is also lit and the side facing away is unlit.  Day and night on the Moon occurs for the same reason it does on the Earth.  There is no permanently "dark side of the Moon" despite Pink Floyd album names.  The only different is that a "day" on the Moon is much longer than a day on the Earth because the Moon rotates on its axis much more slowly.  More on this later.

Anyway, as you can see from the diagram above, when the Moon lies between the Earth and the Sun, the side facing the Sun is lit and the side facing the Earth is unlit.  Also note that such a Moon is high in the sky around noon for the daytime Earth (the side of the Earth that's lit).  Therefore we simply don't see the Moon in the sky.  It's the New Moon phase.

Conversely, when the Moon is on the opposite side of the Earth from the Sun, the side facing the Earth is completely lit and it's high in the night sky around midnight.  Therefore we see it as a Full Moon.

The Moon orbits the Earth in a counterclockwise fashion (as seen from above the North Pole) just as the Earth rotates counterclockwise.  So you can see the 1st Quarter Moon is high in the sky around sunset and from the Earth, you would see the right-hand side of the Moon as lit.  Similarly, the 3rd or Last Quarter Moon would be seen around sunrise and you would see the left-hand side lit (unless you're upside-down in the Southern Hemisphere).

Hopefully, you can also visualize why you see waxing or waning crescent or gibbous moons.  From the perspective of being on the Earth, you will see either less than half the Moon lit (crescent phase) or more than half the Moon lit (gibbous phase).   Note that waxing crescents would be seen in the afternoon and waning crescents in the morning (before noon).  Waxing gibbous is after sunset and waning gibbous before sunrise.  In other words, don't bother looking for a crescent moon high in the sky in the middle of the night.  It will either be setting in the evening after sunset (with the right side lit) or rising in the early morning hours before sunrise (left side lit).

Make sense?  Next we'll talk about how long it takes the Moon to orbit around the Earth.

Thursday, July 14, 2011

Movements of the Moon - Part I

Even though I'm a geologist by training, I do teach courses on Observational Astronomy and Ancient Astronomy (both offered this fall semester, the Observational Astronomy on Thursday nights and the Ancient Astronomy online - contact me if you're interested!).

Teaching these courses has made me much more in tune with the sky and one of the things I always find myself aware of is the phase of the Moon.  As I write this here in Ulster County, for example, it's after midnight and the nearly full Moon is lighting up the night outside.

Over the next few days, I'll be posting some information about the movements of the Moon and I'll start with the most obvious things and then move on to the more subtle movements.  Let's start with the phases of the Moon.

Moon "phases" refer to the percentage and portion of the Moon illuminated by the Sun as seen from the Earth's surface.  A New Moon is not illuminated and invisible.  A Full Moon is 100% illuminated.  The time it takes to go from the New Moon to the Full Moon and back to the New Moon is one cycle of phases and takes about a month (the words "moon" and "month" are obviously cognates).

As the Moon becomes more illuminated, we say it's waxing and as it become less illuminated it's waning.  As the moon waxes, the illumination starts on the right-hand side and progresses from day-to-day until the Moon is full (at least in the Northern Hemisphere).  When the Moon wanes, the the illuminated portion becomes smaller and smaller on the left side of the Moon.

A thin sliver of the Moon is called a crescent, when half the Moon is illuminated we call it a Quarter Moon (1st quarter for waxing and 3rd or last quarter for waning), and when the Moon is mostly illuminated it's gibbous (from the Latin gibbus meaning "hump").  Why quarter Moon when it's half lit?  It's from the fact that they occur at the 1/4 and 3/4 points of the Moon's cycle of phases.

Picture the image below to remember the sequence (read the story behind this).

As I've said, this cycle of phases (called a lunation) takes about a month.  More precisely, it takes 29.53 days (29 days, 12 hours, 44 minutes, and 3 seconds) on average.  Why "on average"?  Because it can vary from lunation to lunation by several hours (approximately 29.18 to 29.93 days) since the Moon's orbit around the Earth is an ellipse and the Earth's orbit around the Sun is also an ellipse and not perfect circles (we'll come back to this later).

If you want to get even more precise, an average lunation takes 29.530588853 + 0.000000002162 * Y days where Y is the number of years since January 1, 2000.  You can see from this formula that the average length of each lunation increases by 2 seconds every 10,000 years (0.000000002162 day/yr * 86,400 sec/day = 0.0001867968 sec/yr).

The time it takes for one lunation is called the synodic month (we'll talk about other "kinds" of months in subsequent posts).  The word synodic, by the way, is derived from the Greek σὺν ὁδῴ (sun hodō) meaning "together with [the Sun]."  This is because Moon phases have to do with the Moon lining up with the Earth and the Sun as we'll see tomorrow.

Lunations are sometimes numbered by a system developed by British mathematician and astronomer Ernest William Brown (1866-1938).  Lunation 1 was the first new Moon in 1923 (0241 UTC, January 17, 1923), the year his lunar tables were incorporated into astronomical almanacs - we're currently in lunation number 1095.

OK, so we know the Moon goes through a cycle of phases - New, Waxing Crescent, 1st Quarter, Waxing Gibbous, Full, Waning Gibbous, 3rd Quarter, Waning Crescent, and New again - in 29.53 days.  Why does it do this?  The full explanation will have to wait until tomorrow.

Wednesday, July 13, 2011

Haven't posted for a while...

I haven't posted for the past week since I've been busy with two online courses which just started (in addition to working around the house, a little hiking, etc.).

In preparation for an Ancient Astronomy course I'll be teaching in the fall, I thought I'd put together a series of posts on the movements of the Moon.  The first one will appear tomorrow.

Tuesday, July 5, 2011

Geocentrism refuses to die for some religious loons

The Chicago Tribune had a story yesterday about a group of religious loons (Catholic, this time) who just can't come to grips with reality (Some Catholics seek to counter Galileo).

You know what?  They're correct.  Hell, they don't even go far enough!  There are many Biblical verses that clearly support a flat, geocentric Earth supported on pillars - here's a list.  This is the real Biblical cosmology that should be supported by all Bible-believing Christians:

If you don't believe this, you're just a heathen in the fast-lane straight to Hell.

Monday, July 4, 2011

Happy Aphelion

Yes, I know, everyone else celebrates the 4th of July.  Don't get me wrong, I'm a huge fan of the freedoms our Constitution and independence gave us, but there's something else that's notable today.

Today, July 4, at 1500 UTC (11:00 am EDT), the Earth will be at aphelion in its elliptical orbit around the Sun.  We're 152,097,700 km (94,509,130 miles) from the Sun today.  Perihelion, when the Earth is closest to the Sun (peri is Greek for "next to" and helios is the Greek word for the Sun), will occur on January 5, 2012 at 0100 UTC.  At that time, we'll be only 147,098,070 km (91,402,500 miles) from the Sun.  This is a difference of about 5 million km (3 million miles).

So, why is it so much hotter today than it will be in early January?  Because the difference in the Earth's position from the Sun between aphelion and perihelion is only about 3% and not really enough to have much of an effect on the amount of insolation (incoming solar radiation) reaching the Earth.  If it were the reason for the seasons, the Northern and Southern Hemispheres would both have summer in January and winter in July.

The real reason for the seasons is the 23.5° tilt of the Earth's axis.  The Northern Hemisphere is tilted toward the Sun on the June solstice giving us more direct sunlight and more hours of sunlight.  In the Southern Hemisphere, they're tilted away from the Sun so it's their winter  On the December solstice, we're tilted away from the Sun so we have less direct sunlight (it comes in at a lower angle) and less hours of sunlight.  The equinoxes, of course, are the half-way points between the solstices.

As we start to move closer and closer to the Sun over the next six months, the Earth will also begin to speed up in its orbital path - all part of Johannes Kepler's Three Laws of Planetary Motion - the subject of another post!

Friday, July 1, 2011

New Nabro Images

New images of Nabro volcano in Eritrea (see my previous post) from the Advanced Land Imager (ALI) on the Earth Observing-1 (EO-1) satellite. The gas and ash cloud has died down a bit allowing a much better view of the volcano and resultant lava flow.

The top image shows the area in visible and infrared light.  The IR indicates heat energy so the vent and hot lava shows up especially well.  Contrast this with the bottom image in natural light in which it's more difficult to see where the hottest material is still flowing.

Since there are no geologists on the ground here (at least that we know of), there is some interest as to the magma composition.  Volcanologist Erik Klemetti has tried to use these satellite images to work this out given how far the magma moved over several days and calculating its viscosity (different magma compositions have different viscosities).